Earth-like planets are common around Sun-like stars, too.

Although NASA's Kepler probe has entered a semi-retirement, discoveries from the data it collected continue. Scientists are currently gathered to discuss these results, and they held a press conference today to announce the latest haul. As of today, the Kepler team is adding 833 new exoplanet candidates to its existing haul, bringing the total up to over 3,500. So far, 90 percent of the candidates that have been checked have turned out to be real. The number of planets in the habitable zone has gone up to over 100.

In conjunction with the press conference, PNAS is releasing a paper that performs an independent analysis of Sun-like stars. This finds that over 20 percent of these host a planet less than two times the size of Earth's radius. Within Kepler's field of view, 10 of them receive an amount of light similar to that reaching Earth.

A status update

Kepler spots planets by watching them transit in front of their host star. This creates a characteristically square-shaped dip in the amount of light reaching Earth. This method of detection, however, isn't considered definitive. The sightings are considered candidates and need to be confirmed by another method.

Based on the amount of light obscured, researchers can estimate the radius of the planet; the time between the transits lets them figure out the orbit. This lets researchers infer the planet's properties, like whether it's likely to be small and rocky like Earth or a Neptune-sized ice or water giant. The orbital period tells us how far the planet is from its host star, which can then be used to determine how much light it receives. In turn, this can indicate whether the planet is within the habitable zone, where liquid water can be present on its surface. (There are, however, a lot of complications that could keep those planets from being habitable.)

Today, to a large extent, was a status report. It was produced by sending more of Kepler's data through its standard analysis pipeline, which looks for the multiple dips caused by repeated transits. With 34 months of data in total, the number of planet candidates has grown to over 3,500, a rise of roughly 30 percent. Although larger planets are easier to spot since they block more light, 600 of these candidates are now Earth-sized or smaller.

As far as the habitable zone goes, we're up to 104 planet candidates. Nearly a quarter of these candidates are two times the Earth's radius or smaller.

Not every exosolar system will be oriented so that its planets will pass between the host star and Earth. As a result, Kepler can only possibly detect a small fraction of planets within its field of view. If you assume that orbit of planets is essentially random, however, you can use statistics to infer the presence of the planets that can't be seen. Performing this process, researchers were able to estimate that 70 percent of main sequence stars (things that aren't white dwarfs or neutron stars) host at least one planet. Half of the M dwarfs, a small but common star, play host to a habitable zone planet.

Looking for Earths

That's all that's coming out of the standard Kepler analysis pipeline. But not satisfied with that, a Berkeley grad student named Erik Petigura went and wrote his own independent pipeline, downloaded the Kepler data, and did his own search. The analysis focused specifically on Sun-like stars, 42,000 of which were present in Kepler's field of view. The software pulled out over 16,000 potential transit events, and the Berkeley team went through and examined each visually to determine whether they were likely to represent planetary candidates.

After they spent a few days blinking to recover from staring at 16,000 brightness curves (note: I made that up), they discovered that their analysis indicated there were 603 planets in the data. They then performed a similar analysis as the Kepler team had, determining how many stars were likely to have planets in a plane that wouldn't lead to transits visible from Earth. The team also identified stars that have activity changes that are so high that they would make it impossible to detect a transit.

When all was said and done, they were able to infer that roughly 20 percent of Sun-like stars have a planet that's two times the radius of Earth or smaller. To be clear: this 20 percent isn't just planets—it's planets that are small and rocky, much like Earth or Venus. Ten of the planets they identified received between one and four times the light that Earth does, which can be extrapolated to similar planets appearing near 10 percent of the Sun-like stars.

The new analysis didn't find a planet in the habitable zone of a Sun-like star, since their signals are harder to pick out, and they take longer to orbit (meaning they've made fewer transits in the Kepler data). Still, the distribution of Earth-sized planets is fairly even closer to the host star, and the authors expect that pattern is likely to stretch into the habitable zone.

Putting all their numbers together, they estimate that there's an Earth-sized planet orbiting a Sun-like star somewhere within 12 light years of our own solar system. Now we just need to figure out how we're going to find it.